Motorcycles have been conventionally designed for use on roadways, trails and race tracks which are relatively free from bumps, ruts, and other obstacles. However, in recent years, off-road use of motorcycles has become increasingly popular, particularly in competitive events such as motocross and desert racing, and motorcycles have been designed specifically for these off road activities, incorporate many improvements over conventional roadway cycle design, including improvements in rear wheel suspension systems. However, these improvements are for the most part modifications of conventional rear suspension units which are comprised of a swinging arm or fork pivoted from the main frame and carrying the rear wheel, and a pair of shock absorbing units connected between the swinging arm and upper members of the main frame.
These suspension systems have proven quite adequate for normal roadway or graded racetrack use. When used at high speeds in rough terrain, they have proven to have many desirable characteristics needed for this use such as high lateral rigidity, low unsprung weight, maintain load on front wheel for steering in turns, and simplicity in construction and servicing. However, they have one major shortcoming in that the sudden vertical shock loads delivered to the rear wheel upon striking an obstacle are absorbed and reacted into the rear of the frame either as a direct vertical force, or a force that has a significant vertical component. This vertical reaction on the rear of the frame causes it to rise suddenly in a rotational motion about the front wheel point of contact with the ground. The result of this motion is loss of traction, and reduced control of the motorcycle, and in severe cases causes loss of rear wheel contact with the ground which may throw the cycle end-over-end into a bad crash. Such accidents are normally avoided by reducing speeds prior to encountering large obstacles when seen sufficiently in advance, but some are not foreseeable, and the desire to achieve competitive racing positions encourages maintaining high speeds when mode rate obstacles are foreseen. Therefore an improvement in rear suspensions for motorcycles used in off-road racing is definitely needed, but must be accomplished in a manner that does not diminish the desirable characteristics inherent in current designs.